Multi-junction solar cell

ABSTRACT

According to one embodiment, a multi-junction solar cell includes a first solar cell, a second solar cell, and an insulating layer. The first solar cell includes a first photoelectric conversion element. The second solar cell is connected in parallel with the first solar cell. The second solar cell includes multiple second photoelectric conversion elements connected in series. The insulating layer is provided between the first solar cell and the second solar cell. The second photoelectric conversion element includes a p-electrode and an n-electrode. The p-electrode is connected to a p+-region including a surface on a side opposite to a light incident surface. The n-electrode is connected to an n+-region including the surface on the side opposite to the light incident surface. The p-electrodes oppose each other or the n-electrodes oppose each other in a region where the multiple second photoelectric conversion elements are adjacent to each other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims the benefit of priorityunder 35 U.S.C. § 120 from U.S. application Ser. No. 14/854,190 filedSep. 15, 2015, and claims the benefit of priority under 35 U.S.C. § 119from Japanese Patent Application No. 2014-191861 filed Sep. 19, 2014;the entire contents of each of which are incorporated herein byreference.

FIELD

Embodiments described herein relate generally to a multi-junction solarcell.

BACKGROUND

There is a multi-junction solar cell that is used as a highly efficientsolar cell. The multi-junction solar cell is, for example, a tandemsolar cell. There are expectations for the multi-junction solar cell tohave a high efficiency compared to a single junction solar cell. On theother hand, a difference undesirably occurs between the current valuesof each layer if the number of photons absorbed by each layer isdifferent.

When the difference between the current values occurs, the conversionefficiency is undesirably limited by the layer having the lowest currentvalue. This is inevitable as long as the layers are connected in series.Conversely, the limit of the conversion efficiency described above canbe avoided by drawing out terminals from each layer. However, it isundesirably necessary to provide multiple power converters, etc. It isdesirable to increase the conversion efficiency of the multi-junctionsolar cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a multi-junctionsolar cell according to an embodiment;

FIG. 2 is a schematic plan view showing the form of element separationaccording to the embodiment;

FIG. 3 is a schematic plan view showing another form of elementseparation according to the embodiment;

FIG. 4 is a schematic plan view showing still another form of elementseparation according to the embodiment;

FIGS. 5A and 5B are schematic plan views showing still another form ofelement separation according to the embodiment;

FIG. 6 is a schematic plan view showing still another form of elementseparation according to the embodiment;

FIG. 7 is a schematic plan view showing still another form of elementseparation according to the embodiment;

FIGS. 8A and 8B are a table and a graph of an example of the measurementresults;

FIG. 9 is a schematic plan view showing still another form of elementseparation according to the embodiment;

FIG. 10 is a schematic plan view showing still another form of elementseparation according to the embodiment;

FIGS. 11A to 11C are schematic plan views showing still another form ofelement separation according to the embodiment; and

FIGS. 12A and 12B are schematic views showing the interconnect patternof the example shown in FIGS. 11A and 11B.

DETAILED DESCRIPTION

According to one embodiment, a multi-junction solar cell includes afirst solar cell, a second solar cell, and an insulating layer. Thefirst solar cell includes a first photoelectric conversion element. Thesecond solar cell is connected in parallel with the first solar cell.The second solar cell includes multiple second photoelectric conversionelements connected in series. The insulating layer is provided betweenthe first solar cell and the second solar cell. The second photoelectricconversion element includes a p-electrode and an n-electrode. Thep-electrode is connected to a p⁺-region including a surface on a sideopposite to a light incident surface. The n-electrode is connected to ann⁺-region including the surface on the side opposite to the lightincident surface. The p-electrodes oppose each other or the n-electrodesoppose each other in a region where the multiple second photoelectricconversion elements are adjacent to each other.

Various embodiments will be described hereinafter with reference to theaccompanying drawings.

The drawings are schematic or conceptual; and the relationships betweenthe thicknesses and widths of portions, the proportions of sizes betweenportions, etc., are not necessarily the same as the actual valuesthereof. The dimensions and/or the proportions may be illustrateddifferently between the drawings, even in the case where the sameportion is illustrated.

In the drawings and the specification of the application, componentssimilar to those described in regard to a drawing thereinabove aremarked with like reference numerals, and a detailed description isomitted as appropriate.

A method (a substrate method) for forming the films on a substrate fromthe p-layer side will now be described as an example. However, similareffects are obtained for a superstrate method as well.

FIG. 1 is a schematic cross-sectional view showing a multi-junctionsolar cell according to the embodiment.

As shown in FIG. 1, the multi-junction solar cell 10 of the embodimentincludes a first solar cell 100, an insulating layer 300, and a secondsolar cell 200. The insulating layer 300 exists between the first solarcell 100 and the second solar cell 200. In the embodiment, atwo-junction solar cell is described as an example. However, themulti-junction solar cell 10 of the embodiment may be a three-junctionsolar cell or higher. The first solar cell 100 is connected in parallelwith the second solar cell 200.

The first solar cell 100 includes one or more first photoelectricconversion elements 110. The first solar cell 100 is the top cell of themulti-junction solar cell 10. The first solar cell 100 shown in FIG. 1has a form in which any number of first photoelectric conversionelements 110 are connected in series. The number of first photoelectricconversion elements 110 is set according to the design. The firstphotoelectric conversion element 110 of the first solar cell 100 isprovided on the insulating layer 300 and includes a lower electrode 111,a photoelectric conversion layer 112 that is provided on the lowerelectrode 111, and an upper electrode 113 that is provided on thephotoelectric conversion layer 112. A not-shown anti-reflection film maybe provided on the upper electrode 113.

Lower Electrode

The lower electrode 111 of the embodiment is an electrode of the firstphotoelectric conversion element 110 and is a conductive film formed onthe insulating layer 300. For example, the conductive film is formed asone body on the insulating layer 300 and subdivided into the lowerelectrodes 111 corresponding to the number of first photoelectricconversion elements 110 by scribing. A film that is conductive andtransparent may be used as the lower electrode 111. Although the lowerelectrode 111 is not limited at all as long as the lower electrode 111is, for example, a transparent conductive film that is transparent andconductive, it is desirable for the lower electrode 111 to include anITO (indium tin oxide ((In, Sn)Ox)) film. In the embodiment, the casewhere the lower electrode 111 (the transparent electrode) is an ITOelectrode is described as an example. However, the lower electrode 111(the transparent electrode) is not limited to the ITO electrode. Thefilm thickness of the lower electrode 111 is, for example, not less than100 nanometers (nm) and not more than 1000 nm. The lower electrode 111is connected to the adjacent upper electrode 113. The multiple firstphotoelectric conversion elements 110 are connected in series with eachother by the connections between the lower electrodes 111 and the upperelectrodes 113.

Photoelectric Conversion Layer

The photoelectric conversion layer 112 of the embodiment is a compoundsemiconductor layer in which a p-type compound semiconductor layer andan n-type compound semiconductor layer have a homojunction, or acompound semiconductor layer in which a p-type compound semiconductorlayer and an n-type buffer layer have a heterojunction. Thephotoelectric conversion layer 112 is formed as one body on the lowerelectrode 111 and subdivided into the photoelectric conversion layers112 corresponding to the number of first photoelectric conversionelements 110 by scribing.

The photoelectric conversion layer 112 converts light into electricityby the compound semiconductor. The p-type compound semiconductor layeris a layer in the region inside the photoelectric conversion layer 112on the lower electrode 111 side. The n-type compound semiconductor layerand the n-type buffer layer are layers in the region inside thephotoelectric conversion layer 112 on the upper electrode 113 side.

For example, a chalcopyrite compound that includes a Group 11 element(Group Ib element), a Group 13 element (Group IIIb element), and a Group16 element (Group VIb element) may be included in the photoelectricconversion layer 112 as the compound semiconductor. The notation of theelement groups conforms to the notation method of the IUPAC(International Union of Pure and Applied Physics). The notation insidethe parentheses is the old notation of the IUPAC.

For example, Cu(In, Al, Ga)(Se, S)₂, CuGaSe₂, CuGa(S, Se)₂, Cu(Ga,Al)Se₂, Cu(Al, Ga)(S, Se)₂ (hereinbelow, called “CIGS” as necessary),etc., may be used as the chalcopyrite compound. Other than thechalcopyrite compound, stannite compounds or kesterite compounds such asCu(Zn, Sn)S₂, Cu(Zn, Sn)(S, Se)₂, Cu(Zn, Sn)Se₂, etc., may be used asthe compound semiconductor of the photoelectric conversion layer 112.Also, a compound semiconductor layer that has a wider gap than thephotoelectric conversion layer of the second solar cell 200 may beincluded in the photoelectric conversion layer 112 of the firstphotoelectric conversion element 110. Generally, the first solar cell100 is not limited at all as long as the number of first photoelectricconversion elements 110 in series is modifiable.

The first photoelectric conversion element 110 includes at least one ofa chalcopyrite compound, a stannite compound, or a kesterite compound.

CdS, etc., may be used as the n-type buffer layer. The compound of thephotoelectric conversion layer 112 expressed as a chemical formula maybe Cu(Al_(w)In_(x)Ga_(1-w-x))(S_(y)Se_(z)Te_(1-y-z))₂,Cu₂ZnSn(S_(y)Se_(1-y))₄, etc. w, x, y, and z satisfy 0≤w≤1, 0≤x≤1,0≤y≤1, 0≤z≤1, w+x≤1, and y+z≤1. The composition of the photoelectricconversion layer 112 can be measured by inductively coupled plasma (ICP)analysis.

The film thickness of the photoelectric conversion layer 112 is, forexample, not less than 1000 nm and not more than 3000 nm. In thephotoelectric conversion layer 112, it is favorable for the filmthickness of the p-type compound semiconductor layer to be not less than1000 nm and not more than 2500 nm. It is favorable for the filmthicknesses of the n-type compound semiconductor layer and the n-typebuffer layer to be not less than 10 nm and not more than 800 nm. It isfavorable to use Cu as the Group 11 element. It is favorable for atleast one type of element selected from the group consisting of Al, In,and Ga to be used as the Group 13 element; and it is more favorable toinclude Ga. It is favorable for at least one type of element selectedfrom the group consisting of O, S, Se, and Te to be used as the Group 16element; and it is more favorable to include Se. It is more favorable touse S as the Group 16 element because the semiconductor can be formed asa p-type semiconductor easily.

Specifically, as the photoelectric conversion layer 112, a compoundsemiconductor such as Cu(Al, Ga)(S, Se)₂, Cu(Al, Ga)(Se, Te)₂, Cu(Al,Ga, In)Se₂, Cu₂ZnSnS₄, etc., and more specifically, a compoundsemiconductor such as Cu(Al, Ga)Se₂, Cu(In, Al)Se₂, CuGaSe₂, CuGa(S,Se)₂, CuAlSe₂, Ag(In, Ga)Se₂, Ag(In, Al)Se₂, Ag(Ga, Al)Se₂, Ag(In, Ga,Al)(S, Se)₂, etc., may be used. It is favorable for a compound thatincludes the elements included in the lower electrode 111 and thephotoelectric conversion layer 112 to exist between the lower electrode111 and the photoelectric conversion layer 112. The effects described inthe embodiment also may be obtained for other solar cells if the solarcell uses the light-transmissive lower electrode 111.

The first solar cell 100 includes the multiple first photoelectricconversion elements 110. One portion of the multiple first photoelectricconversion elements 110 is connected in series with each other. Oneother portion of the multiple first photoelectric conversion elements110 is connected in series with each other. The one portion of themultiple first photoelectric conversion elements 110 is connected inparallel with the one other portion of the multiple first photoelectricconversion elements 110.

Upper Electrode

The upper electrode 113 of the embodiment is a film that is conductiveand transmits light such as sunlight. The upper electrode 113 is formedas one body on the photoelectric conversion layer 112 and subdividedinto the upper electrodes 113 corresponding to the number of firstphotoelectric conversion elements 110 by scribing. The multiple firstphotoelectric conversion elements 110 are connected in series byconnecting the upper electrodes 113 to the lower electrodes 111.

The upper electrode 113 may include, for example, ZnO doped with Al, B,Ga, etc. The upper electrode 113 may be formed as a film by sputtering,chemical vapor deposition (CVD), etc. For example, i-ZnO as asemi-insulating layer may be formed to have a thickness of not less thanabout 10 nm and not more than about 100 nm between the upper electrode113 and the photoelectric conversion layer 112. The semi-insulatinglayer is, for example, a layer including particles of an oxide includingat least one type of element of Zn, Ti, In, Mg, Sn, Ga, Zr, or the like.For example, the particles of the oxide including the elements of Zn andMg are expressed by Zn_(1-x)Mg_(x)O (0≤x≤1).

It is favorable for the average primary particle size of the oxideparticles to be not less than 1 nm and not more than 40 nm. Because theupper electrode 113 is positioned further toward the upper portion thanis the photoelectric conversion layer 112, it is desirable for the upperelectrode 113 to be transparent and to have low sunlight absorptionloss. For example, CdS or Zn(O, S) may be formed to have a thickness ofnot less than about 1 nm and not more than about 10 nm between thesemi-insulating layer and the photoelectric conversion layer 112 recitedabove. This acts to fill the deficiencies of the Group 16 element of thephotoelectric conversion layer 112 and improve the open circuit voltage.Because the film thickness of the CdS or Zn(O, S) is extremely thin,there is substantially no light absorption loss. A window layer may beprovided between the upper electrode 113 and the photoelectricconversion layer 112.

Window Layer (not Shown)

The window layer (not shown) of the embodiment is an i-typehigh-resistance (semi-insulating) layer provided between the upperelectrode 113 and the photoelectric conversion layer 112. The windowlayer is a layer including one compound of ZnO, MgO, (Zn_(a)Mg_(1-a))O,In_(a)Ga_(b)Zn_(c)O, Sn_(a)O, In_(a)Sn_(b)O, TiO₂, or ZrO₂, or a layerincluding up to multiple types of these compounds. It is favorable fora, b, and c to respectively satisfy 0<a<1, 0<b<1, and 0<c<1.

Providing the high-resistance layer between the upper electrode 113 andthe photoelectric conversion layer 112 provides the advantage ofreducing the leakage current from the n-type compound semiconductorlayer toward the upper electrode 113 and increasing the conversionefficiency. The high-resistance compound is included in the compoundincluded in the window layer. Therefore, it is unfavorable for thewindow layer to be too thick. In the case where the film thickness ofthe window layer is too thin, the effect of reducing the leakage currentundesirably is substantially zero. Therefore, the average film thicknessof the window layer is favorably not less than 5 nm and not more than100 nm.

CVD, spin coating, dipping, vapor deposition, sputtering, etc., may beused as the method for forming the window layer.

The oxide thin film of the window layer is obtained by the followingmethod using CVD. The oxide thin film is obtained by introducing theformed members up to the photoelectric conversion layer 112 to achamber, heating to a heated state, further introducing water, anorganic metal compound including at least one of Zn, Mg, In, Ga, Sn, Ti,or Zr, etc., to the chamber, and causing a reaction on the n-typecompound semiconductor layer.

The oxide thin film of the window layer is obtained by the followingmethod using spin coating. A solution including an organic metalcompound or oxide particles including at least one of Zn, Mg, In, Ga,Sn, Ti, or Zr is spin coated onto the formed members up to thephotoelectric conversion layer 112. After the coating, the oxide thinfilm is obtained by heating in a dryer or by causing a reaction.

The oxide thin film of the window layer is obtained by the followingmethod using dipping. The n-type compound semiconductor layer side ofthe formed members up to the photoelectric conversion layer 112 isimmersed in a solution similar to that of spin coating. The members arewithdrawn from the solution after the necessary amount of time. Afterwithdrawing, the oxide thin film is obtained by heating or by causing areaction. The compound thin film of the window layer is obtained by thefollowing method using vapor deposition. Vapor deposition is a methodfor obtaining the oxide thin film by sublimating the window layermaterial by resistance heating, laser irradiation, etc.

Sputtering is a method for obtaining the window layer by irradiatingplasma onto a target.

Among CVD, spin coating, dipping, vapor deposition, and sputtering, thefilm formation methods of spin coating and dipping do not damage thephotoelectric conversion layer and are favorable formation methods fromthe perspective of increased efficiency by not causing recombinationcenters to form in the photoelectric conversion layer.

Intermediate Layer (not Shown)

An intermediate layer (not shown) of the embodiment is a compound thinfilm layer provided between the photoelectric conversion layer 112 andthe upper electrode 113 or between the photoelectric conversion layer112 and the window layer. Although it is favorable for the firstphotoelectric conversion element 110 to include the intermediate layerin the embodiment, the intermediate layer may be omitted. Theintermediate layer is a thin film including one compound of ZnS,Zn(O_(α)S_(β)Se_(1-α-β)), Zn(O_(α)S_(1-α)),(Zn_(β)Mg_(1-β))(O_(α)S_(1-α)), (Zn_(β)Cd_(γ)Mg_(1-β-γ)) (O_(α)S_(1-α)),CdS, Cd(O_(α)S_(1-α)), (Cd_(β)Mg_(1-β))S,(Cd_(β)Mg_(1-β))(O_(α)S_(1-α)), In₂S₃, In₂(O_(α)S_(1-α)), CaS,Ca(O_(α)S_(1-α)), SrS, Sr(O_(α)Si_(1-α)), ZnSe, ZnIn_(2-δ)Se_(4-ε),ZnTe, CdTe, or Si (it being favorable for α, β, γ, δ, and, ε to satisfy0≤α≤1, 0≤β≤1, 0≤γ≤1, 0≤δ≤2, 0≤ε≤4, and β+γ≤1), or a thin film includingup to multiple types of these compounds.

The intermediate layer may have a configuration that does not cover theentire surface of the n-type compound semiconductor layer on the upperelectrode 113 side. For example, it is sufficient to cover 50% of thesurface of the n-type compound semiconductor layer on the upperelectrode 113 side. From the perspective of environmental problems, itis favorable for the compound included in the intermediate layer not toinclude Cd. It is advantageous for the volume resistivity of theintermediate layer to be not less than 1 Ωcm to suppress the leakagecurrent due to low resistance components that may exist inside thep-type compound semiconductor layer. By forming the intermediate layerincluding S, the S that is included in the intermediate layer can bedoped into the n-type compound semiconductor layer.

By including the intermediate layer, the conversion efficiency of thefirst photoelectric conversion element 110 including a homojunction-typephotoelectric conversion layer 112 can be increased. By including theintermediate layer, the open circuit voltage of the first photoelectricconversion element 110 including the photoelectric conversion layer 112having the homojunction structure is increased; and the conversionefficiency can be increased. The role of the intermediate layer is toreduce the contact resistance between the n-type compound semiconductorlayer and the upper electrode 113.

From the perspective of increasing the conversion efficiency, it isfavorable for the average film thickness of the intermediate layer to benot less than 1 nm and not more than 10 nm. The average film thicknessof the intermediate layer is determined from the cross-sectional imageof the first photoelectric conversion element 110. In the case where thephotoelectric conversion layer 112 is the heterojunction-type, a CdSlayer having a thickness of not less than several tens of nm, e.g., 50nm, is necessary as the buffer layer. The intermediate layer is a filmon the n-type compound semiconductor layer and is thinner than thebuffer layer. In the case of the first photoelectric conversion element110 including the heterojunction-type photoelectric conversion layer112, it is unfavorable for the film thickness of the buffer layer to beabout the same as the film thickness of the intermediate layer of theembodiment or for the film thickness of the photoelectric conversionlayer 112 to be about the same as the film thickness of the intermediatelayer of the embodiment because the conversion efficiency decreases.

From the perspective of increasing the conversion efficiency, it isfavorable for the intermediate layer to be a hard film. It is favorableto use chemical bath deposition (CBD), CVD, or physical vapor deposition(PVD) as the method for forming the hard film. The intermediate layermay be a film of an oxide as long as the intermediate layer is a hardfilm. A hard film means that the film is a dense film having a highdensity.

Surface recombination centers undesirably form if the n-type compoundsemiconductor layer is damaged when forming the intermediate layer.Therefore, among the methods recited above, from the perspective oflow-damage film formation, it is favorable for the method for formingthe intermediate layer to be CBD. When making a thin film that is notless than 1 nm and not more than 10 nm, it is sufficient for the growthtime of the film to be short according to the thickness. For example,for the film formation conditions in which a reaction time of 420seconds is necessary to grow an intermediate layer of 60 nm by CBD, forexample, it is sufficient to use a reaction time of 35 seconds to forman intermediate layer of 5 nm. It is also possible to adjust the filmthickness by changing the concentration of the adjusting solution.

Anti-Reflection Film

The anti-reflection film of the embodiment is formed on the upperelectrode 113 and is a film to make it easier to introduce light to thephotoelectric conversion layer 112. For example, it is desirable to useMgF₂ or a microlens (e.g., made by Optmate Corporation) as theanti-reflection film. The film thickness of the anti-reflection film is,for example, not less than 90 nm and not more than 120 nm. For example,the anti-reflection film may be formed by electron beam vapordeposition. In the case where a commercial microlens is used, thethickness of the anti-reflection film is the thickness of the microlens.

It is favorable to provide a rectifying element (a bypass diode) toreduce the effects on the solar cell and the solar cell panel in thecase where breakdown of one first photoelectric conversion element 110of the first solar cell 100 occurs. By providing a bypass diode to beconnected in parallel with each of the first photoelectric conversionelements 110, the effects on the solar cell can be reduced even in thecase where breakdown of one of the multiple first photoelectricconversion elements 110 occurs. It is favorable to connect a bypassdiode to the lower electrode 111 of each of the first photoelectricconversion elements 110 and the upper electrode 113 of each of the firstphotoelectric conversion elements 110. It is favorable for the bypassdiode and the interconnects of the bypass diode to have configurationsthat do not impede the light entering the photoelectric conversion layer112.

A diode may be connected in series with the first solar cell 100. Theanode of the diode that is connected in series with the first solar cell100 is connected to the lower electrode 111 that is used as the negativeterminal of the first solar cell 100. Or, the cathode of the diode isconnected to the upper electrode 113 that is used as the positiveterminal of the first solar cell 100. The diode that is connected inseries with the first solar cell 100 functions to prevent the reverseflow of the electricity when the open circuit voltage of the first solarcell 100 is lower than the open circuit voltage of the second solar cell200.

Similarly to the second solar cell 200, a diode that is connected inseries with the second solar cell 200 may be provided. The anode of thediode connected in series with the second solar cell 200 is connected tothe negative terminal of the second solar cell 200. Or, the cathode ofthe diode is connected to the positive terminal of the second solar cell200. In such a case, the diode that is connected in series with thesecond solar cell 200 functions similarly to the diode connected inseries with the first solar cell.

Diodes that are connected in series to both the first solar cell 100 andthe second solar cell 200 may be provided. When the open circuit voltageof the solar cell decreases due to the failure of one of thephotoelectric conversion elements in the case where the bypass diode isused, the diode that is connected in series functions if the voltagematching between the first solar cell 100 and the second solar cell 200cannot be maintained. The diode that is connected in series causes thevoltage to drop. Therefore, from the perspective of the conversionefficiency, it is favorable for the diode that is connected in series tohave a low voltage drop.

Second Solar Cell

The second solar cell 200 includes multiple second photoelectricconversion elements 210. The second solar cell 200 is used as the bottomcell of the multi-junction solar cell 10. In FIG. 1, the multiple secondphotoelectric conversion elements 210 are connected in series. Thesecond solar cell 200 shown in FIG. 1 has a configuration in which anynumber of second photoelectric conversion elements 210 are connected inseries. The number of second photoelectric conversion elements 210 isset according to the design.

The second solar cell 200 includes, for example, a first photoelectricconversion unit 210 a and a second photoelectric conversion unit 210 b.For example, the second photoelectric conversion unit 210 b is arrangedin a first direction Dr1 with the first photoelectric conversion unit210 a.

For example, one portion of the first solar cell 100 is arranged withthe first photoelectric conversion unit 210 a in a third direction Dr3intersecting the first direction Dr1. For example, one other portion ofthe first solar cell 100 is arranged with the second photoelectricconversion unit 210 b in the third direction Dr3.

The second photoelectric conversion elements 210 are solar cells thatinclude back contacts. The second photoelectric conversion elements 210are provided at the surface of the insulating layer 300 on the sideopposite to the first solar cell 100 as viewed from the insulating layer300. The second photoelectric conversion element 210 includes an n-typeor p-type silicon layer 211, a p⁺-region 212, an n⁺-region 213, ap-electrode 214, and an n-electrode 215. The p⁺-region 212 is providedin a region including the surface of the silicon layer 211 on the sideopposite to the insulating layer 300 (the surface on the side oppositeto the light incident surface) as viewed from the silicon layer 211. Then⁺-region 213 is provided in a region including the surface of thesilicon layer 211 on the side opposite to the insulating layer 300 (thesurface on the side opposite to the light incident surface) as viewedfrom the silicon layer 211. The n⁺-region 213 is provided to beseparated from the p⁺-region 212. The p-electrode 214 is connected tothe p⁺-region 212. The n-electrode 215 is connected to the n⁺-region213.

The first photoelectric conversion unit 210 a includes, for example, afirst electrode unit E1 and a first semiconductor unit S1. The firstsemiconductor unit S1 is provided between the first electrode unit E1and the first solar cell 100. The first semiconductor unit S1 includes afirst region 213 a of the first conductivity type and a second region212 a of the second conductivity type.

The first electrode unit E1 includes, for example, a first electrode 215a and a second electrode 214 a 1. The first electrode 215 a iselectrically connected to the first region 213 a. The second electrode214 a 1 is arranged with the first electrode 215 a in the firstdirection Dr1. The second electrode 214 a 1 is electrically connected tothe second region 212 a.

The second photoelectric conversion unit 210 b includes, for example, asecond electrode unit E2 and a second semiconductor unit S2. The secondsemiconductor unit S2 is provided between the second electrode unit E2and the first solar cell 100. The second semiconductor unit S2 includesa third region 213 b of the first conductivity type and a fourth region212 b of the second conductivity type.

The second electrode unit E2 includes, for example, a third electrode215 b and a fourth electrode 214 b. The third electrode 215 b iselectrically connected to the third region 213 b. The fourth electrode214 b is arranged with the third electrode 215 b in the first directionDr1. The fourth electrode 214 b is electrically connected to the fourthregion 212 b and the first electrode 215 a.

The distance between the second electrode 214 a 1 and the fourthelectrode 214 b is longer than the distance between the first electrode215 a and the third electrode 215 b.

The insulating layer 300 may be provided between the first photoelectricconversion unit 210 a and the first solar cell 100.

The second solar cell 200 further includes an interconnect unit 217. Forexample, the n-electrode 215 (the first electrode 215 a) of the secondphotoelectric conversion element 210 is connected by the interconnectunit 217 to the p-electrode 214 (the fourth electrode 214 b) of thesecond photoelectric conversion element 210. The multiple secondphotoelectric conversion elements 210 are connected in series to eachother. The p⁺-region 212 and the n⁺-region 213 exist at a back contactsurface 221 which is the back surface of the second photoelectricconversion element 210. Although the silicon layer 211 is described asthe n-type in the embodiment, the silicon layer 211 may be the p-type.As described below in regard to FIG. 4 and FIGS. 5A and 5B, aninsulating film 219 may be provided in a region around which thep-electrode 214, the n-electrode 215, and the interconnect unit 217 areprovided. In such a case, the interconnect unit 217 is provided as alead wire or in a film-like configuration.

For example, element separation of the second photoelectric conversionelements 210 of the second solar cell 200 is performed. As shown in FIG.1, the element separation is performed using the interconnect unit 217in the region between the multiple second photoelectric conversionelements 210.

In FIG. 1, double dot-dash lines are drawn between the mutually-adjacentmultiple second photoelectric conversion elements 210 for convenience ofdescription. However, the multiple second photoelectric conversionelements 210 are not subdivided physically. That is, the silicon layer211 is formed as one body.

As the number of regions (the element separation regions) 207 where theelement separation is performed using the interconnect units 217increases, the number of second photoelectric conversion elements 210that can be connected in series increases; and the power generationvoltage of the second solar cell 200 can be increased.

The silicon layer 211 is a p-type or n-type monocrystalline siliconlayer. The film thickness of the silicon layer 211 is, for example, notless than 50 micrometers (μm) and not more than 400 μm. The p⁺-region212 and the n⁺-region 213 are provided in the silicon layer 211. Thesilicon layer 211 includes a dopant such as B, Al, N, P, As, etc. A p-njunction forms between the silicon layer 211 and the p⁺-region 212 orthe n⁺-region 213 to form a photoelectric conversion layer. Ananti-reflection film may be provided between the silicon layer 211 andthe insulating layer 300.

The p⁺-region 212 is, for example, a region where p-type (p⁺) ionimplantation of the silicon layer 211 is performed; and the p⁺-region212 is formed to include the surface (the back surface) on the sideopposite to the insulating layer 300 as viewed from the silicon layer211. The n⁺-region 213 is, for example, a region where n-type (n⁺) ionimplantation of the silicon layer 211 is performed; and the n⁺-region213 is formed to include the surface (the back surface) on the sideopposite to the insulating layer 300 as viewed from the silicon layer211.

The p⁺-region 212 and the n⁺-region 213 have configurations that aresimilar to each other such as a U-shaped configuration, a comb-shapedconfiguration, etc. The p⁺-region 212 is provided on the backside of thep-electrode 214 (the silicon layer 211 side). The n⁺-region 213 isprovided on the backside of the n-electrode 215 (the silicon layer 211side). The p⁺-region 212 is disposed to mesh with the n⁺-region 213. Thep⁺-region 212 does not contact the n⁺-region 213. It is favorable for aregion of the silicon layer 211 to exist between the p⁺-region 212 andthe n⁺-region 213.

The second region 212 a and the first region 213 a have comb-shapedconfigurations. The second region 212 a is disposed to mesh with thefirst region 213 a.

For example, the ion implantation is performed by using a mask anddoping the silicon layer 211 with a dopant such as B, Al, N, P, As,etc., so that the p⁺-region 212 or the n⁺-region 213 is formed in aregion having a depth of not less than 50 nm and not more than 2 μm (aregion between the back contact surface 221 and a location in the rangeof not less than 50 nm and not more than 2 μm from the back contactsurface 221 into the interior of the silicon layer 211). It is favorablefor the dopant concentration of each region to be not less than about1.0×10¹⁸ cm⁻³ and not more than about 1.0×10²⁰ cm⁻³. The dopantconcentrations of the p⁺-region 212 and the n⁺-region 213 are higherthan the impurity concentration of the silicon layer 211.

From the cost perspective, other than ion implantation, it is favorableto use thermal diffusion, etc., as the method for forming the p⁺-region212 and the n⁺-region 213.

The p⁺-region 212 is connected to the p-electrode 214. The n⁺-region 213is connected to the n-electrode 215. The p-electrode 214 and then-electrode 215 are used as the back contact electrodes of the secondsolar cell 200. The p-electrode 214 and the n-electrode 215 are used aselectrodes to connect the multiple second photoelectric conversionelements 210 in parallel or in series. In the case where the bypassdiode is provided, a diode is connected to at least one of thep-electrode 214 or the n-electrode 215. The p-electrode 214 and then-electrode 215 are, for example, a Cu or Al film deposited using a maskto have a thickness of about 1 μm.

If breakdown of one second photoelectric conversion element 210 of thesecond solar cell 200 occurs, it is favorable to provide a bypass diodeto reduce the effects on the solar cell and the solar cell panel. Byproviding the bypass diode connected in parallel with each of the secondphotoelectric conversion elements 210, the effects on the solar cell canbe reduced even when the breakdown of one of the multiple secondphotoelectric conversion elements 210 occurs. It is favorable to connectthe bypass diode to the p-electrode 214 of each of the secondphotoelectric conversion elements 210 and the n-electrode 215 of each ofthe second photoelectric conversion elements 210. The diode may beformed by ion implantation, etc., of the silicon layer 211; or the diodemay be connected externally.

The multi-junction solar cell and the form of element separation willnow be described with reference to the drawings.

FIG. 2 is a schematic plan view showing the form of element separationaccording to the embodiment.

FIG. 2 is a schematic plan view when the multi-junction solar cellaccording to the embodiment is viewed in the direction along arrow Alshown in FIG. 1.

The first solar cell 100 may be one element or may have a structure inwhich multiple elements are connected in series. For example, even inthe case where the silicon layer 211 is formed as one body, it issufficient for the second photoelectric conversion elements 210 to beperceived as one cell.

FIG. 2 is a conceptual view of a multi-junction solar cell 10 a that hasthe form of element separation in which the second solar cell 200 isdivided into four second photoelectric conversion elements 210. In themulti-junction solar cell 10 a of FIG. 2, the mutually-adjacent secondphotoelectric conversion elements 210 are substantially electricallydisconnected from each other. In other words, in the element separationregion 207, the mutually-adjacent second photoelectric conversionelements 210 are electrically isolated from each other but notphysically separated from each other. In a general back contact-typesilicon solar cell, one p-electrode 214 and one n-electrode 215 exist asone set. For example, if a general back contact-type silicon solar cellwere to be described using the example shown in FIG. 2, one p-electrode214 of the multiple p-electrodes 214 disposed on the leftmost side andone n-electrode 215 of the multiple n-electrodes 215 disposed on theleftmost side would exist as one set; and the silicon layer 211 would bedivided physically in the element separation region 207 on the leftside. In such a case, for example, the interconnect unit 217 would beprovided as a lead wire. According to the embodiment, the process ofsubdividing every cell of conventional manufacturing is unnecessary; andit is also favorable for the manufacturing in that it is unnecessary tohandle cells subdivided into small pieces.

First Example

In a first example, the multi-junction solar cell 10 a is made to havethe form shown in the conceptual view of FIG. 2. First, the first solarcell 100 is made on soda-lime glass which is used as a portion of theinsulating layer 300. 1 cm by 1 cm of soda-lime glass on which an ITOfilm (the lower electrode 111) having a thickness of 200 nm is formed isused; and a Cu_(0.85)(In_(0.12)Ga_(0.59)Al_(0.29))(S_(0.1)Se_(0.9))₂thin film that is used to form the photoelectric conversion layer 112 isdeposited by vapor deposition (a three-stage method). First, thesubstrate temperature is increased to 300° C.; and Al, In, Ga, S, and Seare deposited (the first stage). Subsequently, the substrate temperatureis increased to 500° C.; and Cu, S, and Se are deposited. The start ofthe endothermic reaction is confirmed; and the depositing of the Cu isstopped so that the composition has excessive Cu (the second stage).Directly after stopping the deposition, Al, In, Ga, S, and Se again aredeposited (the third stage) so that a Cu-deficient state is formed and acomposition having excessive Group 13 elements such as Al, In, Ga, etc.,is formed. The film thickness of the photoelectric conversion layer 112is about 2000 nm.

Because a portion of the photoelectric conversion layer 112 that isobtained is to be of the n-type, the deposited members up to thephotoelectric conversion layer 112 are immersed in 25% ammonia solutionin which a concentration of 0.08 mM of cadmium sulfate is dissolved; anda reaction is performed for 22 minutes at room temperature (25° C.).Thereby, an n-type semiconductor layer doped with Cd is formed to adepth of about 100 nm in the region on the side used to form the upperelectrode 113 of the photoelectric conversion layer 112. A CdS contactlayer is deposited on the n-type semiconductor layer by spin coating ani-ZnO thin film which is a semi-insulating layer. Continuing, about 300nm of alumina (Al₂O₃) that is used to form the upper electrode 113 isdeposited on the semi-insulating layer by sputtering using a ZnO:Altarget containing 2 wt % of alumina. Finally, the first solar cell 100is made on the soda-lime glass by depositing about 105 nm of MgF₂ as ananti-reflection film by electron beam vapor deposition.

Then, the second solar cell 200 is made. p⁺-type and n⁺-type ionimplantation is performed into portions of one surface of an n-typemonocrystalline silicon layer having a thickness of 200 μm to formseparate regions having a concentration of 2.0×10¹⁹ cm⁻³, a depth of 0.2μm, and a width of 300 μm and into which the element of B or the elementof P is implanted. By the p⁺-type and n⁺-type ion implantation, twop⁺-regions 212 and two n⁺-regions 213 are formed in the order of p⁺, n⁺,p⁺, and n⁺ on the back contact surface 221 side (referring to FIG. 1) ofthe silicon layer 211. Then, two second photoelectric conversionelements 210 are formed. FIG. 2 shows an example in which four secondphotoelectric conversion elements 210 are formed. A region where ionimplantation is not performed exists between the p⁺-region 212 and then⁺-region 213. Using a mask, 1 μm of Cu is deposited as the p-electrode214 on the p⁺-region 212. Using a mask, 1 μm of Cu is deposited as then-electrode 215 on the n⁺-region 213.

For two mutually-adjacent second photoelectric conversion elements 210as shown in FIG. 2, the n-electrode 215 of one of the twomutually-adjacent second photoelectric conversion elements 210 isconnected by the interconnect unit 217 to the p-electrode 214 of theother of the two mutually-adjacent second photoelectric conversionelements 210. The element separation is performed using the interconnectunit 217 provided in the region between the two mutually-adjacent secondphotoelectric conversion elements 210. Thereby, the multiple secondphotoelectric conversion elements 210 are connected in series with eachother.

The electrodes that have the same polarity are provided on two sides ofthe element separation region 207 and oppose each other. For example, inthe example shown in FIG. 2, the p-electrodes 214 that are on the twosides of the element separation region 207 at the center oppose eachother. Also, in the example shown in FIG. 2, the n-electrodes 215 opposeeach other for the element separation region 207 on the left side andthe element separation region 207 on the right side. Thus, the secondsolar cell 200 is made.

It is favorable for a distance D1 between the electrodes to be short forthe electrodes that have the same polarity and oppose each other in theelement separation region 207. However, leakage current may exist in thecase where the distance D1 between the electrodes is shorter than aprescribed distance. Therefore, in the embodiment, the distance D1between the electrodes is set to a distance such that the leakagecurrent does not exist.

Then, the silicon layer 211 of the second solar cell 200 is adhered tothe soda-lime glass including the first solar cell 100 using an acrylicresin as a bonding agent so that the surface of the silicon layer 211 onthe side opposite to the surface including the n⁺-region 213 and thep⁺-region 212 is adhered to the surface of the soda-lime glass on theside opposite to the surface including the first solar cell 100. Thethickness of the layer of the bonding agent is about 50 μm.

The ITO electrode and the upper electrode 113 of the first solar cell100 and the p⁺-region 212 and the n⁺-region 213 were connected to asemiconductor parameter analyzer; and using artificial sunlightirradiation of AM 1.5 by a solar simulator, the open circuit voltage(Voc), the short circuit current (Isc), and the conversion efficiency(η) were measured for the solitary first solar cell 100, the solitarysecond solar cell 200 in the state in which the first solar cell 100 isformed, and the multi-junction solar cell in which the first solar cell100 and the second solar cell 200 are connected in parallel. Themeasurement results are shown in Table 1. A conversion efficiency η iscalculated from the formula η=Voc·Jsc·FF/P·100 using an open circuitvoltage Voc, a short circuit current density Jsc, an output factor FF,and an incident power density P. Because 1 cm by 1 cm of soda-lime glass(having a surface area of 1 cm²) is used in the first example, the shortcircuit current (Isc) is equal to the short circuit current density(Jsc). This is similar for the first comparative example describedbelow.

First Comparative Example

In the first comparative example, other than the element separation notbeing performed, the multi-junction solar cell is made similarly to thatof the first example; that is, for two mutually-adjacent secondphotoelectric conversion elements 210 such as those shown in FIG. 2, then-electrode 215 of one of the two mutually-adjacent second photoelectricconversion elements 210 is not connected by the interconnect unit 217 tothe p-electrode 214 of the other of the two mutually-adjacent secondphotoelectric conversion elements 210. Similarly to the first example,the open circuit voltage, the short circuit current, and the conversionefficiency was measured for the multi-junction solar cell of the firstcomparative example as well. The measurement results are shown inTable 1. In Table 1, the properties of the first solar cell CELL1, thesecond solar cell CELL2 and the multi-junction solar cell MCELL areshown with respect to the first example EX1 and the first comparativeexample CP1.

TABLE 1 CELL1 CELL2 MCELL Voc (V) Isc (mA) η (%) Voc (V) Isc (mA) η (%)Voc (V) Isc (mA) η (%) EX1 1.2 16 15.3 1.3 12 12.1 1.2 27 24.6 CP1 1.216 15.3 0.7 21 10.3 0.7 35 18.4

As shown in Table 1, in the first example, the voltage of the secondsolar cell 200 is improved and is about twice that for the case wherethe connection in series (the element separation) is not made (the firstcomparative example). In the first example, the current value of thesecond solar cell 200 is about half of that for the case where theconnection in series (the element separation) is not made (the firstcomparative example). Thereby, the target performance is realized.

In the first comparative example, a voltage increase of the second solarcell 200 substantially is not obtained. This is because the structure isnot a serially-connected structure because the element separation is notperformed. In the first comparative example, because the pattern of theconfiguration of the back surface electrodes is different from that whenconnected in series, the efficiency is undesirably low because thecurrent value is not increased sufficiently because some of the photonsthat are absorbed undesirably recombine. Comparing the results of thefirst example and the results of the first comparative example, it canbe understood that voltage matching cannot be performed if the elementseparation is not performed. That is, in the multi-junction solar cell10 a according to the embodiment, the conversion efficiency can beincreased by performing the element separation of the second solar cell200 and by performing voltage matching between the first solar cell 100and the second solar cell 200.

The matching is not limited to voltage matching; and current matchingalso is possible. In other words, current matching is possible bysubdividing the elements to equalize the amount of photons absorbed percell surface area instead of subdividing the elements to equalize thevoltages.

As another comparative example, the measurement results of the opencircuit voltage, the short circuit current, and the conversionefficiency of the solitary second solar cell 200 in the state in whichthe first solar cell 100 is not formed are shown in Table 2. In Table 2,the properties of the first solar cell CELL1, the second solar cellCELL2 and the multi-junction solar cell MCELL are shown with respect tothe second example EX2 and the second comparative example CP2.

TABLE 2 Voc(V) Isc(mA) η (%) 0.7 38 20.1

Comparing the conversion efficiency shown in Table 2 to the conversionefficiency of the multi-junction solar cell shown in Table 1, theconversion efficiency of the multi-junction solar cell of the firstexample is higher than the conversion efficiency of the solitary secondsolar cell 200 in the state in which the first solar cell 100 is notformed. On the other hand, the conversion efficiency of themulti-junction solar cell of the first comparative example is lower thanthe conversion efficiency of the solitary second solar cell 200 in thestate in which the first solar cell 100 is not formed. Thus, in the casewhere the multi-junction solar cell is made simply without performingvoltage matching between the first solar cell 100 and the second solarcell 200, the conversion efficiency of the multi-junction solar cell maybe lower than the conversion efficiency of the solitary second solarcell 200.

FIG. 3 is a schematic plan view showing another form of elementseparation according to the embodiment.

FIG. 3 shows the conceptual view and the circuit of a multi-junctionsolar cell 10 b.

Second Example

The ITO electrode (the lower electrode 111) is subdivided intoelectrodes for twenty elements on a 12 cm by 12 cm substrate byscribing; and the photoelectric conversion layer 112 is deposited on theITO electrode. The photoelectric conversion layer 112 is scribed so thatthe first photoelectric conversion element 110 is subdivided into twentyequal parts; the upper electrode 113 is formed and scribed so that thetwenty first photoelectric conversion elements 110 are connected inseries; and subsequently, an anti-reflection film is formed to make thefirst solar cell 100 in which twenty first photoelectric conversionelements 110 are connected in series.

Then, thirty-eight p⁺-regions 212 and thirty-eight n⁺-regions 213 thatare connected in series are formed by ion implantation in a 12 cm by 12cm silicon layer 211; and thirty-eight second photoelectric conversionelements 210 having equal surface areas are made. This structure isbonded to the substrate in which the first solar cell 100 is formed; andthe multi-junction solar cell 10 b is made similarly to the firstexample. The measurement results are shown in Table 3.

Second Comparative Example

In a second comparative example, the silicon solar cell of thefoundation is not connected in series. That is, the element separationof the second solar cell 200 is not performed. Otherwise, themulti-junction solar cell is made similarly to the second example. Themeasurement results are shown in Table 3.

TABLE 3 CELL1 CELL2 MCELL Voc (V) Isc (mA) η (%) Voc (V) Isc (mA) η (%)Voc (V) Isc (mA) η (%) EX2 24.5 112 15.2 24.7 91.5 12.4 24.5 203 27.3CP2 24.5 112 15.2 0.7 2500 12.3 0.7 2600 12.8

In the second example, the number of second photoelectric conversionelements 210 of the second solar cell 200 connected in series is higher.Also, the difference between the voltage of the first solar cell 100 andthe voltage of the second solar cell 200 is smaller. Therefore, in thesecond example, compared to the first example, a highly efficientmulti-junction solar cell is obtained; and the second example iseffective.

FIG. 4 is a schematic plan view showing still another form of elementseparation according to the embodiment.

FIGS. 5A and 5B are schematic plan views showing still another form ofelement separation according to the embodiment.

In the example shown in FIG. 4, a multi-junction solar cell 10 c is madesimilarly to the first example. As shown in FIG. 4, the n-electrode 215of one of two mutually-adjacent second photoelectric conversion elements210 is connected by the interconnect unit 217 to the p-electrode 214 ofthe other of the two mutually-adjacent second photoelectric conversionelements 210. Thereby, the multiple second photoelectric conversionelements 210 are connected in series with each other.

The insulating film 219 is provided between the interconnect unit 217and the silicon layer 211. In the case where the conductivity betweenthe interconnect unit 217 and the silicon layer 211 is low, theinsulating film 219 may not always be provided.

For example, in the case where the p-electrode 214 and the n-electrode215 oppose each other from two sides of the element separation region207, leakage current occurs due to the shape of the electric fielddistribution; and the effect of the p-n junction decreases. Conversely,in the example shown in FIG. 4, the p-electrodes 214 oppose each otherfrom the two sides of the element separation region 207. Thereby, thedecrease of the effect of the p-n junction can be suppressed.

In the case where the insulating film 219 is provided between theinterconnect unit 217 and the silicon layer 211, for example, thep⁺-region 212 and the p-electrode 214 may pass through on the lower sideof the interconnect unit 217 as in a multi-junction solar cell 10 dshown in FIG. 5A and FIG. 5B. Thereby, the leakage current that occursbetween the p-electrode 214 and the n-electrode 215 on the two sides ofthe element separation region 207 can be suppressed further.

The region that passes through on the lower side of the interconnectunit 217 may be the n⁺-region 213; and the electrode that passes throughon the lower side of the interconnect unit 217 may be the n-electrode215.

The second photoelectric conversion unit 210 b further includes thesilicon layer 211 and the insulating layer 300. The silicon layer 211includes the third region 213 b and the fourth region 212 b. Theinsulating layer 300 is provided between the interconnect unit 217 andthe silicon layer 211.

At least a portion of at least one of the third electrode 215 b or thefourth electrode 214 b is positioned between the interconnect unit 217and the first solar cell 100.

FIG. 6 is a schematic plan view showing still another form of elementseparation according to the embodiment.

In the example shown in FIG. 6, a multi-junction solar cell 10 e is madesimilarly to the first example. As shown in FIG. 6, the n-electrode 215of one of two mutually-adjacent second photoelectric conversion elements210 is connected by the interconnect unit 217 to the p-electrode 214 ofthe other of the two mutually-adjacent second photoelectric conversionelements 210.

Here, for example, the multiple second photoelectric conversion elements210 are connected by the interconnect unit 217 not only in the firstdirection Dr1 (in FIG. 6, the horizontal direction) but also in a seconddirection Dr2 (in FIG. 6, the vertical direction) intersecting the firstdirection Dr1 and intersecting the third direction Dr3.

For example, the second solar cell 200 further includes a thirdphotoelectric conversion unit 210 c and a fourth photoelectricconversion unit 210 d. For example, the third photoelectric conversionunit 210 c is arranged with the fourth photoelectric conversion unit 210d in the first direction Dr1. The third photoelectric conversion unit210 c is arranged with the second photoelectric conversion unit 210 b inthe second direction Dr2. The fourth photoelectric conversion unit 210 dis arranged with the first photoelectric conversion unit 210 a in thesecond direction Dr2.

For example, the first photoelectric conversion unit 210 a and thesecond photoelectric conversion unit 210 b are connected in series. Forexample, the second photoelectric conversion unit 210 b and the thirdphotoelectric conversion unit 210 c are connected in series. Forexample, the third photoelectric conversion unit 210 c and the fourthphotoelectric conversion unit 210 d are connected in series.

Thereby, in the example shown in FIG. 6, four second photoelectricconversion elements 210 (the first photoelectric conversion unit 210 a,the second photoelectric conversion unit 210 b, the third photoelectricconversion unit 210 c, and the fourth photoelectric conversion unit 210d) are connected in series with each other.

For example, the second solar cell 200 is multiply provided. Themultiple second solar cells 200 are arranged along the second directionDr2. One of the multiple second solar cells 200 is connected in serieswith one other of the multiple second solar cells 200.

In the example shown in FIG. 6 as well, the electrodes having the samepolarity oppose each other from the two sides of the element separationregion 207. For example, in the example shown in FIG. 6, thep-electrodes 214 oppose each other from the two sides of the elementseparation region 207.

FIG. 7 is a schematic plan view showing still another form of elementseparation according to the embodiment.

FIGS. 8A and 8B are a table and a graph of an example of the measurementresults.

In the example shown in FIG. 7, a multi-junction solar cell 10 f is madesimilarly to the first example. In the example shown in FIG. 7, thelength of an electrode 214 a of one of the electrodes having the samepolarity opposing each other from the two sides of the elementseparation region 207 is changed. As shown in FIG. 7, for example, thelength of the p-electrode 214 a disposed on the left side of the elementseparation region 207 is changed.

A ratio LR (%) of the length of the p-electrode 214 a to a length L1 ofone side of the silicon layer 211 is set to be 50, 60, 70, 80, 90, and100. The state of the ratio LR being 100 is the state in which thesilicon layer 211 is completely divided by the p-electrode 214 a (thestate in which the silicon layer 211 is actually covered with thep-electrode 214 a) when the silicon layer 211 is viewed from theelectrode side.

A 12 cm by 12 cm wafer is used as the silicon layer 211. The shortcircuit current density Jsc of the solitary silicon layer 211 is 38mA/cm². The open circuit voltage Voc of the solitary silicon layer 211is 0.7 V. The theoretical limit of the short circuit current density Jscof the state in which the two second photoelectric conversion elements210 are connected (the state in which the silicon layer 211 iscompletely divided) is set to the value (19 mA/cm²) of ½ of the shortcircuit current density of the solitary silicon layer 211. Thetheoretical limit of the open circuit voltage Voc is set to the value(0.7 V) of the open circuit voltage of the solitary silicon layer 211.

In the case of the multi-junction solar cell 10 f of FIG. 7, the shortcircuit current density Jsc, the open circuit voltage Voc, the outputfactor FF, the conversion efficiency η (%), and the ratio η/η_(Id) (%)of the conversion efficiency η to the theoretical limit of theconversion efficiency η_(Id) (%) are measured by setting the ratio LR(%) of the p-electrode 214 of one of the second photoelectric conversionelements 210 of the mutually adjacent p-electrodes 214 of the secondphotoelectric conversion elements 210 to 100% and changing the ratio LR(%) of the p-electrode 214 of the other second photoelectric conversionelement 210. In the measurement, the n-electrode 215 of the one of thesecond photoelectric conversion elements 210 is connected to thep-electrode 214 of the other second photoelectric conversion element210. The measurement results are shown in FIG. 8A and FIG. 8B.

As shown in FIG. 8A and FIG. 8B, the ratio η/η_(Id) (%) increases as theratio LR (%) increases. That is, the conversion efficiency η approachesthe theoretical limit of conversion efficiency η_(Id) as the state inwhich the two second photoelectric conversion elements 210 are connected(the state in which the silicon layer 211 is completely dividedelectrically) is approached.

FIG. 9 is a schematic plan view showing still another form of elementseparation according to the embodiment.

In the example shown in FIG. 9, a multi-junction solar cell 10 g is madesimilarly to the first example. Based on the measurement resultsdescribed above in regard to FIG. 7 to FIG. 8B, the p-electrode 214 hasan arrangement pattern provided around the periphery of the n-electrode215 in the example shown in FIG. 9. In other words, the p-electrode 214has an encircling electrode pattern in which the electrode is disposedto completely encircle. The n-electrode 215 may have the encirclingelectrode pattern instead of the p-electrode 214.

One of the second electrode 214 al or the first electrode 215 a isprovided around at least a portion of the other of the second electrode214 a 1 or the first electrode 215 a.

As shown in FIG. 9, the n-electrode 215 of one of two mutually-adjacentsecond photoelectric conversion elements 210 is connected by theinterconnect unit 217 to the p-electrode 214 of the other of the twomutually-adjacent second photoelectric conversion elements 210. Thereby,the two second photoelectric conversion elements 210 are connected inseries with each other. As described above in regard to FIG. 4 and FIGS.5A and 5B, the insulating film 219 may be provided between theinterconnect unit 217 and the silicon layer 211. In such a case, theinterconnect unit 217 is provided as a lead wire or in a film-likeconfiguration.

In the example shown in FIG. 9, the conversion efficiency can beincreased further.

The case where the p-electrode 214 or the n-electrode 215 has theencircling electrode pattern is shown in FIG. 9. The p-electrode 214 orthe n-electrode 215 may have a comb-shaped electrode pattern instead ofthe encircling electrode pattern. Even in such a case, the conversionefficiency can be increased.

In the case where the surface areas of the encircling electrode patternsare equal, currents having substantially the same current value flow inthe second photoelectric conversion elements 210. Therefore, in the casewhere the surface areas of the encircling electrode patterns are equal,the number of subdivided second photoelectric conversion elements 210can be set arbitrarily. For example, it is unnecessary for thesubdivided number to be evenly divisible; and the subdivided number maybe 3 columns up by 3 columns across, 2 columns up by 5 columns across,etc. For example, the subdivided number may be 11, etc.

FIG. 10 is a schematic plan view showing still another form of elementseparation according to the embodiment.

An ITO electrode (the lower electrode 111) is subdivided into electrodesfor four elements on a 2 cm by 2 cm substrate by scribing; and thephotoelectric conversion layer 112 is deposited on the ITO electrodes. Adimension L2 of the ITO electrode (referring to FIG. 1) is, for example,about 5 millimeters (mm) due to the relationship with the mobility ofthe photoelectric conversion layer 112, etc. The dimension L2 of the ITOelectrode is equal to the dimension of the width of the firstphotoelectric conversion element 110. A chalcopyrite compound of CuGa(S,Se)₂ (hereinbelow, called “CGSS” as necessary) is used as thephotoelectric conversion layer 112.

The photoelectric conversion layer 112 is scribed so that the firstphotoelectric conversion element 110 is subdivided into four equalparts; the upper electrode 113 is formed and scribed so that the fourfirst photoelectric conversion elements 110 are connected in series; andan anti-reflection film is formed to make the first solar cell 100 inwhich the four first photoelectric conversion elements 110 are connectedin series.

Then, seven p⁺-regions 212 and seven n⁺-regions 213 are formed by ionimplantation into a 2 cm by 2 cm silicon layer 211; and seven secondphotoelectric conversion elements 210 having equal surface areas aremade. This structure is bonded to the substrate in which the first solarcell 100 is formed; and a multi-junction solar cell 10 h is madesimilarly to the first example.

The open circuit voltage Voc that is measured for the solitary firstsolar cell 100 is 4.8 volts (V). The short circuit current density Jscthat is measured for the solitary first solar cell 100 is 4milliampere/square centimeter (mA/cm²).

On the other hand, the open circuit voltage Voc that is measured for thesolitary second solar cell 200 in the state in which the first solarcell 100 is formed is 4.7 (V). The short circuit current density Jscthat is measured for the solitary second solar cell 200 in the state inwhich the first solar cell 100 is formed is 3.4 (mA/cm²).

The conversion efficiency η is 27(%); and a multi-junction solar cellhaving a relatively high conversion efficiency is obtained.

In the example, the case is assumed where the bandgap of the first solarcell 100 is 1.9 electron volts (eV).

The open circuit voltage Voc that is measured for the solitary secondsolar cell 200 in the state in which the first solar cell 100 is formedis 4.7 (V). The short circuit current density Jsc that is measured forthe solitary second solar cell 200 in the state in which the first solarcell 100 is formed is 2.9 (mA/cm²). On the other hand, in the case wherethe first photoelectric conversion element 110 is subdivided into fiveequal parts, the open circuit voltage Voc that is measured for thesolitary first solar cell 100 is 5.0 (V). The short circuit currentdensity Jsc that is measured for the solitary first solar cell 100 is 4(mA/cm²).

The conversion efficiency η is 26(%).

FIGS. 11A to 11C are schematic plan views showing still another form ofelement separation according to the embodiment.

FIGS. 12A and 12B are schematic views showing the interconnect patternof the example shown in FIGS. 11A and 11B.

FIG. 11A is a schematic plan view showing the first solar cell 100according to the embodiment. FIG. 11B is a schematic plan view showingthe second solar cell 200 according to the embodiment. FIG. 11C is aschematic plan view showing a second solar cell 200 a according to areference example. FIG. 12A is a schematic cross-sectional view showingthe interconnect pattern of the example shown in FIGS. 11A and 11B. FIG.12B is a schematic plan view showing the interconnect pattern of theexample shown in FIGS. 11A and 11B.

The second solar cell 200 is not shown in FIG. 12A.

FIG. 11A to FIG. 12B show the case where the substrate is panel-sized asan example.

More specifically, the case where twelve 12 cm by 12 cm silicon layers211 are used is described as an example.

In such a case as shown in FIG. 11A, the length of one side of the firstsolar cell 100 is 400 mm. The length of the other side of the firstsolar cell 100 is 500 mm.

As described above in regard to FIG. 10, in the case where CGSS is usedas the photoelectric conversion layer 112, the dimension L2 of the ITOelectrode (the dimension of the width of the first photoelectricconversion element 110) is, for example, about 6 mm due to therelationship with the mobility of the photoelectric conversion layer112, etc. Therefore, because the length of the one side of the firstsolar cell 100 is 400 mm, sixty-six first photoelectric conversionelements 110 are connected in series in the first solar cell 100.

Here, because the number of elements that can be connected in series inone silicon wafer is limited, it is also possible to connect the firstsolar cells 100 in parallel.

In other words, as shown in FIG. 12A and FIG. 12B, two groups ofthirty-three first photoelectric conversion elements 110 connected inseries are formed; and the two groups are connected in parallel to eachother. At a boundary portion 120 shown in FIG. 12A and FIG. 12B, themutually-adjacent first photoelectric conversion elements 110 areelectrically isolated from each other.

Three groups of twenty-two first photoelectric conversion elements 110connected in series may be formed; and the three groups may be connectedin parallel to each other. One panel may be connected in series; or twosmaller panels may be prepared and connected in series.

Thus, by connecting the first solar cells 100 in parallel, the number ofsecond photoelectric conversion elements 210 connected in series in onesilicon wafer can be reduced.

As shown in FIG. 12B, in the case where two groups of thirty-three firstphotoelectric conversion elements 110 connected in series are formed andthe two groups are connected in parallel to each other, five p⁺-regions212 and five n⁺-regions 213 are formed by ion implantation into thesilicon layer 211; and five second photoelectric conversion elements 210that are connected in series and have equal surface areas are made. InFIG. 11B and FIG. 11C, the second solar cells 200 and 200 a at the upperleft have forms in which five second photoelectric conversion elements210 having equal surface areas are connected in series. The form inwhich the five second photoelectric conversion elements 210 having equalsurface areas are connected in series is the same for the second solarcells 200 and 200 a other than those of the upper left as well.

As shown in FIG. 11B, for example, three of the silicon layers 211 arearranged in the first direction; and four of the silicon layers 211 arearranged in the second direction. Three silicon layers 211 are connectedin series in the first direction. Also, the three silicon layers 211connected in series are connected in parallel in the second direction.That is, twelve silicon layers 211 are connected to be three in seriesby four in parallel.

In other words, a group of the three silicon layers 211 connected inseries in the first direction is connected in parallel in the seconddirection with another group of the three silicon layers 211 connectedin series in the first direction. The first solar cell 100 and thesecond solar cell 200 are connected in parallel.

Here, as in the reference example shown in FIG. 11C, an example isconsidered in which twelve silicon layers 211 are connected in series.When a shadow 401 exists on the silicon layer 211 of at least one of thetwelve silicon layers 211, the output of the second solar cell 200 aexisting under the shadow 401 is substantially zero. Therefore, becausethe twelve silicon layers 211 are connected in series, the output of alltwelve second solar cells 200 a is substantially zero.

Conversely, in the example shown in FIG. 11B, as described above, thetwelve silicon layers 211 are connected to be three in series by four inparallel. Therefore, the output of the region existing under the shadow401 is substantially zero; but the output is not substantially zero forthe regions where the shadow 401 does not exist. Thereby, the outputdecreases only by the amount of the ratio of the surface area of theregion where the shadow 401 exists to the surface area of all of thetwelve silicon layers 211. In other words, other than the case where theshadow 401 exists on all of the twelve silicon layers 211, the output ofall of the twelve second solar cells 200 becoming substantially zerowhen the shadow exists on the silicon layer 211 of one of the twelvesilicon layers 211 can be suppressed. In the example shown in FIG. 11B,it is more desirable to provide a diode to prevent the reverse flow ofelectricity.

In the embodiment, a multi-junction (tandem) solar cell is described asan example. However, the solar cell may not be tandem and may have astructure in which solitary silicon is connected.

In the example shown in FIG. 11A, FIG. 11B, FIG. 12A, and FIG. 12B, ashort circuit current Isc of the multi-junction solar cell 10 i is 1.56(A). The open circuit voltage Voc of the multi-junction solar cell 10 iis 40 (V). The conversion efficiency q of the multi-junction solar cell10 i is 24.3(%).

Embodiments include following Clauses:

Clause 1

A multi-junction solar cell, comprising:

a first solar cell; and

a second solar cell connected in parallel with the first solar cell,

the second solar cell including

-   -   a first photoelectric conversion unit, and    -   a second photoelectric conversion unit arranged in a first        direction with the first photoelectric conversion unit,

one portion of the first solar cell being arranged with the firstphotoelectric conversion unit in a third direction intersecting thefirst direction,

one other portion of the first solar cell being arranged with the secondphotoelectric conversion unit in the third direction,

the first photoelectric conversion unit including

-   -   a first electrode unit, and    -   a first semiconductor unit provided between the first electrode        unit and the first solar cell,

the first semiconductor unit including

-   -   a first region of a first conductivity type, and    -   a second region of a second conductivity type,

the first electrode unit including

-   -   a first electrode electrically connected to the first region,        and    -   a second electrode electrically connected to the second region        and arranged with the first electrode in the first direction,

the second photoelectric conversion unit including

-   -   a second electrode unit, and    -   a second semiconductor unit provided between the second        electrode unit and the first solar cell,

the second semiconductor unit including

-   -   a third region of the first conductivity type, and    -   a fourth region of the second conductivity type,

the second electrode unit including

-   -   a third electrode electrically connected to the third region,        and    -   a fourth electrode electrically connected to the fourth region        and the first electrode and arranged with the third electrode in        the first direction,

a distance between the second electrode and the fourth electrode beinglonger than a distance between the first electrode and the thirdelectrode.

Clause 2

The cell according to Clause 1, further comprising an insulating layerprovided between the first photoelectric conversion unit and the firstsolar cell.

Clause 3

The cell according to Clause 1, wherein the second solar cell furtherincludes an interconnect unit electrically connecting the firstelectrode to the fourth electrode.

Clause 4

The cell according to Clause 1, wherein

the second region and the first region have comb-shaped configurations,and

the second region is disposed to mesh with the first region.

Clause 5

The cell according to Clause 1, wherein one of the second electrode orthe first electrode is provided around at least a portion of one otherof the second electrode or the first electrode.

Clause 6

The cell according to Clause 3, wherein

the second photoelectric conversion unit includes:

-   -   a silicon layer including the third region and the fourth        region; and    -   an insulating film provided between the interconnect unit and        the silicon layer, and

at least a portion of at least one of the third electrode or the fourthelectrode is positioned between the interconnect unit and the firstsolar cell.

Clause 7

The cell according to Clause 1, wherein a plurality of the second solarcells are provided, the second solar cells are arranged along a seconddirection intersecting the first direction, and one of the second solarcells is connected in series with one other of the second solar cells.

Clause 8

The cell according to Clause 1, wherein

the second solar cell further includes a third photoelectric conversionunit and a fourth photoelectric conversion unit,

the third photoelectric conversion unit is arranged with the fourthphotoelectric conversion unit in the first direction,

the third photoelectric conversion unit is arranged with the secondphotoelectric conversion unit in a second direction intersecting thefirst direction and intersecting the third direction,

the fourth photoelectric conversion unit is arranged with the firstphotoelectric conversion unit in the second direction,

the first photoelectric conversion unit and the second photoelectricconversion unit are connected in series,

the second photoelectric conversion unit and the third photoelectricconversion unit are connected in series, and

the third photoelectric conversion unit and the fourth photoelectricconversion unit are connected in series.

Clause 9

The cell according to Clause 1, wherein

the first solar cell includes a plurality of first photoelectricconversion elements, and

the first photoelectric conversion elements are connected in series.

Clause 10

The cell according to Clause 9, wherein the first photoelectricconversion elements include a photoelectric conversion layer, and thephotoelectric conversion layer includes CuGa(S, Se)₂.

Clause 11

The cell according to Clause 1, wherein

the first solar cell includes a plurality of the first photoelectricconversion elements,

one portion of the first photoelectric conversion elements is connectedin series with each other,

one other portion of the first photoelectric conversion elements isconnected in series with each other, and

the one portion of the first photoelectric conversion elements isconnected in parallel with the one other portion of the firstphotoelectric conversion elements.

Clause 12

The cell according to Clause 9, wherein

the first photoelectric conversion elements include at least one of achalcopyrite compound, a stannite compound, or a kesterite compound.

Although several embodiments of the invention are described, theseembodiments are presented as examples and are not intended to limit thescope of the invention. These novel embodiments may be implemented inother various forms; and various omissions, substitutions, andmodifications can be performed without departing from the spirit of theinvention. Such embodiments and their modifications are within the scopeand spirit of the invention and are included in the invention describedin the claims and their equivalents.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the invention.

What is claimed is:
 1. A multi-junction solar cell, comprising: a firstsolar cell including a first photoelectric conversion element; a secondsolar cell connected in parallel with the first solar cell, the secondsolar cell including a plurality of second photoelectric conversionelements connected in series with each other; an insulating layerprovided between the first solar cell and the second solar cell; and aninterconnect portion, one of the second photoelectric conversionelements being closest to an other one of the second photoelectricconversion elements among the second photoelectric conversion elements,a direction from the one of the second photoelectric conversion elementstoward the other one of the second photoelectric conversion elementsbeing along a first direction, the one of the second photoelectricconversion elements including a first region of a first conductivitytype, a second region of a second conductivity type, a first electrodeconnected with the first region, the first electrode including firstfinger parts extending along a second direction crossing the firstdirection, and a second electrode connected with the second region, thesecond electrode including second finger parts and a first partconnected with the second finger parts, the second finger partsextending along the second direction, one of the second finger partsbeing between one of the first finger parts and an other of the firstfinger parts, the other of the first finger parts being between the oneof the second finger parts and an other of the second finger parts, thefirst part being provided around the first electrode in a planeincluding the first direction and the second direction, the other one ofthe second photoelectric conversion elements including a third region ofthe first conductivity type, a fourth region of the second conductivitytype, a third electrode connected with the third region, the thirdelectrode including third finger electrodes extending along the seconddirection, and a fourth electrode connected with the fourth region, thefourth electrode including fourth finger parts and a second partconnected with the fourth finger parts, the fourth finger partsextending along the second direction, one of the fourth finger partsbeing between one of the third finger parts and an other of the thirdfinger parts, the other of the third finger parts being between the oneof the fourth finger parts and an other of the fourth finger parts, thesecond part being provided around the third electrode in the plane, theinterconnect portion connecting the one of the first finger parts withthe second parts; a part of the interconnect portion overlaps at least apart of the first part in a direction crossing the plane.
 2. Themulti-junction solar cell according to claim 1, wherein the first partcircumscribes the first electrode, and the second part circumscribes thethird electrode.
 3. The multi-junction solar cell according to claim 1,wherein the first part encloses the first electrode, and the second partencloses the third electrode.
 4. The multi-junction solar cell accordingto claim 1, wherein the other one of the second photoelectric conversionelements further includes an insulating film provided between theinterconnect portion and the at least a part of the first part.
 5. Themulti-junction solar cell according to claim 1, wherein one portion ofthe plurality of second photoelectric conversion elements is connectedin series in the first direction, and one other portion of the pluralityof second photoelectric conversion elements is connected in series in another direction different from the first direction.
 6. Themulti-junction solar cell according to claim 1, wherein the first solarcell includes multiple first photoelectric conversion elements, and themultiple first photoelectric conversion elements are connected inseries.
 7. The multi-junction solar cell according to claim 6, wherein aphotoelectric conversion layer included in the first photoelectricconversion element includes CuGa(S, Se)₂, and a group of the multiplefirst photoelectric conversion elements connected in series is connectedin parallel with another group of the multiple first photoelectricconversion elements connected in series.
 8. The multi-junction solarcell according to claim 1, wherein the first photoelectric conversionelement includes at least one of a chalcopyrite compound, a stannitecompound, or a kesterite compound.
 9. The multi-junction solar cellaccording to claim 1, wherein the first region is located between thefirst electrode and the insulating layer, the second region is locatedbetween the second electrode and the insulating layer, the third regionis located between the third electrode and the insulating layer, and thefourth region is located between the fourth electrodes and theinsulating layer.
 10. The multi-junction solar cell according to claim1, wherein the first region, the second region, the third region, andthe fourth region extend in the second direction.
 11. The multi-junctionsolar cell according to claim 1, wherein the second finger parts fitwith the first finger parts, and the fourth finger parts fit with thethird finger parts.